The present invention relates to process variable transmitters of the type used to sense a process variable in an industrial process. More specifically, the present invention relates to process variable transmitters used to sense a remote pressure of a process fluid of an industrial process.
Process variable transmitters are used in industrial processes to sense process variables. Examples of process variables include temperature, flow rate, pressure, etc. The process variable transmitter senses a process variable and transmits information related to the process variable to a centralized location, for example, a central process control room. The sensed process variable can be used to monitor operation of the process and, in some instances, can be used to control operation of the process. Various techniques are used for the transmission of the process variable including both wired and wireless techniques. One well known wired technique is a two-wire process control loop in which an analog current level is used to represent the process variable. For example, a 4 mA current level can represent a low value of a process variable and a 20 mA current level can represent a high value. The same two-wire process control loop can also be used to power the process variable transmitter.
One type of process variable transmitter is used to sense level of a process fluid in a tank or container. U.S. Pat. No. 5,899,962, issued May 4, 1999, entitled “DIFFERENTIAL PRESSURE MEASUREMENT ARRANGEMENT UTILIZING DUAL TRANSMITTERS,” and assigned to Rosemount Inc. shows one example of such a configuration and is incorporated herein by reference in its entirety. For example, a process variable transmitter can use two pressure sensors which are vertically spaced apart. A differential pressure between the two sensors can be used as an indication of the amount of process fluid contained in the container. In one configuration, the process pressures are physically coupled to a differential pressure sensor in the process variable transmitter which is used to measure the differential pressure. However, this requires a mechanical coupling through a conduit or “fill tube” which can lead to inaccuracies and also may require relatively large openings (“penetrations”) through walls and the like for the conduit. Further, the use of a mechanical coupling of the two pressures to a differential pressure sensor can introduce errors into the pressure measurement, for example, due to temperature variations. In another configuration, such as that set forth in U.S. Pat. No. 5,899,962, the two pressures are measured separately, and electronically communicated to circuitry which calculates the differential pressure. However, in some environments, it is not possible to use digital circuitry, for example in nuclear power plant applications. The harsh radiation environment may damage such circuitry.